Method for manufacturing cold-rolled steel sheet

ABSTRACT

A METHOD OF MANUFACTURING COLD-ROLLED STEEL SHEET COMPRISING STEPS OF PRODUCING INGOTS FROM A MOLTEN STEEL COMPOSED MAINLY OF 0.06 TO 0.15% C AND 0.10 TO 0.25% MN, TO WHICH LESS THAN 0.06% CU AND 0.015 TO 0.035% P ARE INCLUDED AND MORE OVER AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OF V, NB, AL, TI AND ZR IS INCLUDED IN SMALL AMOUNT, IF DESIRED, WHILE REGULATING NON-METALLIC INCLUSION CLEANLINESS OF THE STEEL SHEET TO 0.2% TO 0.5% AND SUBJECTING A STEEL SHEET OBTAINED THROUGH HOT-ROLLING AND COLD-ROLLING TO A DECRABURIZING ANNEALING TO REDUCE THE CARBON CONTENT OF 0.002 TO 0.015%.   D R A W I N G

Aug. 10, 1971 KAMEO MATSUKURA ETAL 3,598,658

METHOD FOR MANUFACTURING COLD-ROLLED STEEL SHEET Filed May 20, 1968 0.1 0.2 0.3 0.4 0.5 0.6 Degree of Non-Metallic Inclusion Cleanliness d 60x400(%) Number of F ishscole Occurrence 10x15 cm 1N VE N TOPS Kameo Mafsukura Masami O zaw0 United States Patent Oifiee 3,598,658 Patented Aug. 10, 1971 ABSTRACT OF THE DISCLOSURE A method of manufacturing cold-rolled steel sheet comprising steps of producing ingots from a molten steel composed mainly of 0.06 to 0.15% C and 0.10 to 0.25% Mn, to which less than 0.06% Cu and 0.015 to 0.035% P are included and more over at least one element se lected from the group consisting of V, Nb, Al, Ti and Zr is included in small amount, if desired, while regulating non-metallic inclusion cleanliness of the steel sheet to 0.2 to 0.5% and subjecting a steel sheet obtained through hot-rolling and cold-rolling to a decarburizing annealing to reduce the carbon content of 0.002 to 0.015%.

The present invention relates to a method for manufacturing cold-rolled steel sheet, and more particularly a method for manufacturing a deep drawing steel sheet having excellent cold press formability and an enameling steel sheet having no boiling and fishscaling of the enamel coat.

The main object of the present invention is to provide a cold-rolled steel sheet, the cold press formability (primary cold press formability and secondary cold workability) and particularly the primary and secondary cold workability of which is not deteriorated even with a high non-metallic inclusion index for such steel sheet.

The second object of the present invention is to provide an enameling cold-rolled steel sheet which is not softened after an enamel-firing.

The third object of the present invention is to provide a homogeneous one-coat or two-coat enameling coldrolled steel sheet, in which surface defects such as boiling and fishscales are prevented from occurring during and after enamel firing.

The fourth object of the present invention is to provide an enameling cold-rolled steel sheet which is capable of being rapidly pickled in a short time when subjected to a pickling as a pretreatment for enameling.

Other objects will be made clear from the following description and attached drawing.

The attached drawing shows the relation between the degree of cleanliness of a steel sheet or a non-metallic inclusion index, indicating the degree of freedom of the steel from non-metallic inclusions (hereinafter it will be referred to as the degree of non-metallic inclusion cleanliness) and occurrence of fishscales at a certain pickling loss.

The above-mentioned objects of the present invention can be obtained, when the fundamental chemical composition of the steel at the time of tapping is 0.06 to 0.15 Wt. percent C, 0.10 to 0.25 wt. percent Mn and balance being Fe and unavoidable impurities, the degree of nonmetallic inclusion cleanliness (d 60 x 400 according to Japanese Industrial Standard) is kept at a range from 1 (d 60 x 400 according to Japanese Industrial Standard) designates a cleanliness of :1 steel obtained by observing a sample by means 01' JL microscope of -10!) magnifications, while changing the observing spots of the sample times. It is equal to the point counting method of ASTM.)

0.2 to 0.5%, and there are included therein, as occasion demands, less than 0.06 wt. percent Cu, 0.015 to 0.035 wt. percent P and further at least one element selected from the group consisting of 0.02 to 0.05 wt. percent V, 0.02 to 0.05 wt. percent Nb, 0.01 to 0.08 wt. percent A], 0.01 to 0.20 wt. percent Ti and 0.01 to 0.50 wt. percent Zr, and an ingot made from the steel having the abovementioned composition is further subjected to hot-rolling, cold-rolling and then decarburizing annealing to such a degree that the carbon content is 0.002 to 0.015 wt. per cent.

Heretofore, when forming various wares from a thin steel sheet by pressing the same, particularly when carrying out an ultra deep drawing, it is usually necessary to subject the steel sheet to press working two times or more, that is, to so-called multiple stage drawing, because the pressworking cannot be completed by pressing the steel sheet only one time. However, in the case of forming a very deep tumbler-shaped ware or trimming away ends of a cup (uneven ends due to so-called caring) by using a trimmer or subjecting a trimmed ware to a curling or a flange bending working, there has often been perceived brittle rupture occurring along the axial direction of the ware, e.g., in its longitudinal direction, thereby giving rise to troubles. This rupture is characterized in that it has no connection with the rolling direction of the steel sheet, that is, it occurs only in the axial direction of the pressed parts, but not in the circumferential direction, and the fracture is not due to a ductile rupture but a brittle fracture, which occurs along grain boundaries. Further, there is a general tendency for it to occur more in a cold winter season than in a summer season. This rupture is generally designated as a secondary working defect in the industrial circles concerned. In the case of a coldrolled steel sheet, this secondary working defect often appears particularly in a steel material which was subjected to an open-coil decarburizing annealing. As the result of various investigations made heretofore for clearing up its causes a method has been worked out, based on the idea that oxidation of the grain boundaries on the surface of a steel sheet is caused at the time of decarburizing annealing by the decomposition of H 0 contained in the annealing atmosphere, whereby the brittle rupture is easily caused, and in the method an effort was made to forcibly expel oxygen, which entered the grain boundaries, by inserting a drying step after the decarburizing annealing. For this purpose an atmospheric gas of an extremely low dewpoint was maintained at almost the same temperature as the decarburizing annealing temperature for several hours after the completion of the decarburizing annealing, resulting in a prolongation of the annealing time. However, even this method was attended with a defect, because many products treated by this method were not free from secondary working cracks.

The inventors of the present invention have found out that by regulating the carbon content of a molten steel prepared in a steelmaking furnace such as an oxygen top blowing converter, open-hearth furnace or electric furnace, and also the non-metallic inclusion cleanliness of the steel sheet to a specified range respectively, the press formability is not deteriorated and the formation of fishscales after porcelain enameling is very slight.

That is, by regulating the carbon content of a steel prepared in a steelmaking furnace such as converter, openhearth furnace or electric furnace to be below 0.15% and above 0.06%, the best secondary workability of the steel can be achieved. In general, in order to obtain an excellent press formability (deep drawing workability, stretch formability, bending workability and the like), a steel should be tapped in as low a carbon range as possible, and practically one strives to reduce the carbon content as much as possible (below 0.05%), particularly when an excellent press formability is required, though it is, of course, not possible to lower excessively the carbon content of the steel as tapped in view of the oxygen content of the molten steel and the rimming action and the like.

However, as above-mentioned, the inventors of the present invention have found out that the carbon content of a molten steel as tapped has a great influence on the secondary workability of a steel sheet made from the molten steel, that is, the secondary workability of the steel sheet is somewhat deteriorated, if the carbon content of the molten steel as tapped is too low. They have ascertained that the lower limit of the carbon content should be 0.06%, that is, the carbon content should be higher than 0.06%. However, if the carbon content is unnecessarily increased, there is the danger that the oxygen content of the molten steel will be reduced too much, whereby the degree of non-metallic inclusion cleanliness will become lower than the specified range. On the other hand, when the carbon content exceeds 0.15%, the necessary decarburizing time in an open ooil annealing must be undesirably lengthened. Therefore, in order to attain the objects of the present invention the carbon content of the steel as tapped should be in the range of 0.06 to 0.15%, preferably in the range of 0.09 to 0.15%.

According to the present invention, though the oxygen content of the molten steel is reduced on account of the carbon content thereof being raised, provision is made for increasing the oxygen content by reducing the manganese content to a range of 0.10 to 0.25% in order to counterbalance the said reduction of the oxygen content. As well known, manganese is used as a deoxidizer, though it is weak, and is further added for the purpose of preventing the occurrence of a hot-rolling brittleness when subjecting the steel to a hot-rolling. In this case the addition of manganese is usually in a range of 0.25 to 0.50%. However, in the present invention the manganese content is reduced to a range of 0.10 to 0.25%, as already mentioned, which is very helpful for remarkably improving the secondary workability of steel. Moreover, as a result of tests made on the relation between the manganese content and press formability properties it has been confirmed that also the primary forming workability can be remarkably improved by the reduction in the manganese content, which is assumed to be attributable to the fact that very fine grains of non-metallic inclusions such as MnO or MnS would favorably act on the formation of anypreferred orientation texture desirable for a press formation at the time of their recystallization after coldrolling.

Further, it is also well known that a firing strain or sagging which occurs during enamel firing can largely be reduced by the reduction in the manganese content. Therefore, not only from the viewpoint of the cold press workability but also from that of the enamelability it is desirable to specify the manganese content to be in the range as abovementioned.

Still further it was confirmed that, if the manganese content is below 0.10% and the sulfur content is high, there occurs a red shortness when carrying out a hotrolling in a hot strip mill with the unfavorable result such as edge cracks in the rolled sheet, but if the sulfur content is below 0.012%, there occur no such edge cracks without carrying out a delayed rolling at the time of hotrolling, even though the manganese content is in the range of 0.10 to 0.25%. In view of the primary workability, the degree of non-metallic inclusion cleanliness and the red shortness as above-mentioned the manganese content of 0.12 to 0.20% seems to be the most desirable, provided that the sulfur content remains in a normal range.

Further, in the present invention solid non-metallic inclusions are greater than in a usual rimmed steel and the degree of non-metallic inclusion cleanliness (d 60 x 400) is made to be in the range of 0.2 to 0.5%. As is generally known, these non-metallic inclusions are classified into following three series: A series inclusions such as sulfide and silicate, in which plastic deformation is brought about at hot-rolling by the application of mechanical forces (the A series inclusions are further classified into A series, to which sulfide belongs and A series to which silicate belongs); B series inclusions include oxides such as A1 0 which are of granular form and are aligned in discontinuous arrangement, forming groups in the direction of applied mechanical working; and C series inclusions such as A1 0 and/or TiO which are granular oxides and are irregularly dispersed without plastic deformation being brought about. In the steel sheet according to the present invention at least one kind of these series of inclusions must be retained, and a better result can be obtained by the presence of the A series inclusions than the A series inclusions.

It has been brought to light that between these nonmetallic inclusions and the steel matrix, after cold-rolling there are produced micro voids or cracks (owing to the difference in ductility between the non-metallic inclusions and the steel at the temperatures of hot-rolling and particularly cold-rolling) and these void spaces play an important role as a good repository for hydrogen absorbed from an atmosphere at the time of enamel-firing. Therefore, it is desirable to make an ingot in the form of a capped steel, in order to cause thereby the non-metallic inclusions to uniformly disperse in the ingot in adequate sizes and within the predetermined content range. In this case the production of ingot may be carried out either according to a method of producing mechanical capped steel or a method of producing chemical capped steel. In the case of the chemical capped steel, however, a better result can be obtained by using silicon rather than by using aluminum.

Other accompanying elements such as silicon and sulfur and the like may be included in substantially the same amounts as in the usual cold-rolled steel sheet. However, as already mentioned, in order to prevent the formation of fishscales when applying an enamel coat, the index of non-metallic inclusion cleanliness is to be kept above 0.20%. If the cleanliness index of the steel sheet is below 0.20%, therefore, it is necessary to throw into a mold during the process of teeming an additive of foreign matter, which may produce nonmetallie inclusions, for instance, a tablet or pill prepared by wetmixing adequately pulverized slag, scum, silica or ferrochrome etc. together with cut wires used for shot blasting, thereby to positively increase the non-metallic inclusions, particularly silicate, until the cleanliness index is caused to go above 0.20%. In this case, the silicon content of the molten steel becomes sometimes less than 0.03%.

As the result of experiments made by the inventors of the present invention it has been confirmed that, if the index of non-metallic inclusion cleanliness is maintained in the present invention, while non-metallic inclusions are uniformly dispersed in the steel, mechanical properties of the steel are not noticeably deteriorated as compared with the usual rimmed steel having a cleanli ness index of 0.10 to 0.15%. Thus, it is possible to secure good primary formability as well as good secondary formability. The reason why the cleanliness index was specified in the present invention to be in the range of 0.20 to 0.50% is as above-mentioned. But, the preferred range is the range of 0.20 to 0.40%.

The sulfur content is preferably rather low in view of the fact that the manganese content is being reduced.

Phosphorus is added in an amount of 0.015 to 0.035% for the purpose of improving resistance to fishscale formation during porcelain enameling, pickling loss velocity and enamel adhesiveness. However. the most preferable amount is 0.020 to 0.030% to obtain good primary workability and secondary workability. It is unquestionable that the pickling loss velocity may be improved by the addition of phosphorus. But, in a comparative test made on a steel material of low and high phosphorus content, in which both materials were subjected to an enameling with one-coat, while regulating the pickling time so that the same pickling loss was produced, it was further observed that fewer fishscales were formed in the steel material of high phosphorus content. The reason therefor is still unknown, but it can well be presumed that phosphorus itself has the property of improving the resistance to fishscale formation of a steel.

The addition of copper is to be limited to below 0.06% for improving the pickling loss velocity and enamel adhesiveness. The smaller the copper content, the more desirable. But, taking steelmaking conditions and raw material conditions into consideration it is most preferable to limit the copper content to below 0.05%.

Besides the above-mentioned elements, following elements may be further selectively added with a view to improving the secondary workability and the pickling loss and preventing the softening of an enameled steel sheet after an enamel firing due to an abnormal growth of crystal grains at the time of enameLfiring, that is, at least one element selected from the group consisting of 0.02 to 0.05% V, 0.02 to 0.05% Nb, 0.01 to 0.20% Ti, 0.01 to 0.08% Al and 0.01 to 0.50 Zr. V, Nb, Zr and Ti are added in the metallic form or in the form of their ferro alloys, and Al in the metallic form. The inventors of the present invention can not explain the reasons Why the addition of these elements has the effect of improving the secondary workability and other properties. However, they have succeeded in confirming the facts that by the addition of vanadium not only the secondary workability is improved but also the pickling loss velocity is increased by to and further by the selective addition of these elements the growing and coarsening of crystal grains due to strain annealing at the time of enamel-firing after the steel is subjected to a pressforming is largely suppressed.

In the process of manufacturing the steel sheet as a final product of the present invention there are following steps to be passed through after the production of ingot and the slabbing thereof, that is, hot-rolling-picklingcold-rolling-annealing.

The hot-rolling of the steel sheet is carried out according to a conventional method, only with the exception of a coiling temperature particularly specified by the present invention. When hot-rolling the steel of the present invention on a hot strip mill, delayed rolling is not required as in the case of an industrial pure iron, and the finishing temperature of the hotaolled strip coil is sufiicient if it is in the range of 840 to 950 C., as is usually adopted. The preferred finishing temperature is 870 to 900 C. On the other hand, it is absolutely necessary to limit the coiling temperature to the range above 680 C. and below 750C. The most preferred temperature is in the range of 700 to 750 C. By coiling the hot-rolled coil at such a high temperature as above-mentioned, carbon contained in the steel becomes huge amoebashaped carbide when the hot-rolled coil is slowly cooled down from the above-mentioned high temperature in the form of a coil tightly coiled, and this carbide is finely pulverized when the steel is subjected to the subsequent cold-rolling, whereby there can be produced in the steel spaces sufiicient for storing therein hydrogen which is an origin of the fishscale formation. In cases in which the coiling temperature can not be raised to such a high temperature as above-mentioned or the coiling has been carried out at a low temperature, the hot-rolled coil should be reheated to a temperature in the range of 720 to 750 C. and then cooled slowly. In this way the same effect can be achieved as in the case of the coiling at the specified temperature, because the same phenomenon occurs. Further, it has been perceived that the high coiling temperature has a tendency of accelerating the pickling loss velocity at the time of pickling which is to be carried out as a pretreatment for enamel coating.

In regard to the pickling of hot-rolled strip coil it is not an essential feature of the present invention, whether sulfuric acid or hydrochloric acid is applied.

The cold-rolling is a process step of reducing the thickness of a pickled coil down to the final gauge of the product. As the result of the investigations made by the inventors of the present invention it is preferred to adopt a reduction rate near the upper limit within the range of 50 to in the interests of increasing the resistance to fishscale formation at the time of enamel firing and improving the secondary workability. The cold-rolling reduction rate is determined by the relation between the thickness of a hot-rolled steel sheet and that of a coldrolled steel sheet. However, in view of the foregoing it is clear that in this case the hot-rolled steel sheet should be made as thick as possible. Further, it is noted that a reduction rate which exceeds the upper limit of the said range is not desirable because of lack of resistance to fishscale formation at the time of enamel coating, and a reduction rate below the lower limit is also undesirable because of the reduction of the strength of enameled parts after enamel-firing or the press formability of the steel sheet.

In the last step of the decarburizing annealing the decarburization is carried out by using an open-coil decarburizing annealing device and annealing to such a degree that the carbon content of the final steel sheet is in a range of 0.002 to 0.015%.

The decarburization is carried out for the purpose of preventing occurrence of boiling or a formation of blister at the time of enameling with one-coat and improving the primary press forming workability. In the case of manufacturing a steel sheet to be used for parts, in which the secondary workability problem is particularly serious, it is necessary to turn out a final product by confining the decarburization to a somewhat higher halfway decarburization or a surface layer decarburization so that the average carbon content of the product is in the range of 0.006 to 0.015%, instead of carrying out the decarburization down to the lower limit (0.002 to 0.005% C). By doing so, a remarkable improvement in the secondary workability can be obtained, as was ascertained through various experiments.

In the foregoing an enameling steel sheet has been the main subject of the explanation. However, the steel sheet obtained by the method of the present invention is also useful as a steel sheet which is to be chemically treated or coated with organic paints or metal-plated or chemical-synthetically treated after press forming, without carrying out the final enameling, and the steel sheet manufactured by the method of the present invention is accordingly not limited to an enameling steel sheet only but also includes a cold-rolled steel sheet in general within a scope not deviating from the spirit of the present invention.

In the following examples the present invention shall be explained.

EXAMPLE 1 A steel having a chemical composition as shown in Table l was prepared by melting in an oxygen top blowing converter, and ingots were produced therefrom as rimmed steel and mechanical capped steel. A slab obtained by slabbing was rolled to a hot-rolled coil of 2.3 mm. on a hot strip mill, wherein the finishing temperature was 890 C. and the coiling temperature was 700 C. After pickling the hot-rolled coil was cold-rolled at a reduction rate of 68% to produce a cold-rolled coil of 0.8 mm. Then, the thus obtained cold-rolled coil was subjected to a decarburizing annealing in an open-coil annealing furnace at a temperature of 750 C. for 5 hours.

A comparison of the fishcale occurrence of the mechanical capped steels which are the object of the present invention with rimmed steels used as a reference is also shown in the same table.

A test was further made on the relation between pickling loss and number of fishscale occurrence, While keeping the degree of non-metallic inclusion cleanliness at nearly constant values. Therefore, this test was carried out only on the mechanical capped steel. The results are shown in Table 2.

In connection with Table 2 it is noted that the attached drawing shows the relation between the degree of nonmetallic inclusion cleanliness and number of fishscales occurring at a fixed pickling loss, indicating that the occurrence of fishscales becomes very low, if the degree of nonmetallic inclusion cleanliness exceeds 0.2% and scarcely varies after exceeding 0.3%

TABLE 1 However, if the degree of cleanliness of a steel exceeds 0.5%, the mechanical properties of the steel deteriorate and the steel is contaminated too much thereby.

EXAMPLE 2 [Chemical compositions and relations between fishscale occurrence and degree of non-metallic inclusion cleanliness oi ditierent kinds of enameling steel sheets] Number of fishscale Degree occurrence at the time 01-- of non- I metallic Twtrcoat One-coat Ladle analysis (percent) inclusion enameling enameling clean per 10 x 16 of steel Kind of steel Mn Si P S liness l cm. sheet 0. 0. 29 0. 01 0. 014 0. 014 0. 17 132 138 Itinuned steel 0. 06 0. 31 0. 01 0. 012 0. 011 0. 18 218 215 0. 08 0.32 0. 01 0. 011 0. 016 0. 227 220 0. 0B 0. 20 0. 01 0. 023 0. 013 0. 27 20 Mechanical capped steel 0.07 0. 17 0. 01 0. 021 0.012 0. 13 0 0. 08 0. 12 0. 01 0. 027 0. 008 0. 0 1

1 (Central part of steel sheet) (1 x 400. 2 Subjected to open-coil decarbnrizing annealing.

NOTE:

2. Number of fishscale occurrence indicates number of fishscales one side only.

TABLE 2 [Relation between pickling loss and number 1. As a result of the open-coil decarburizing annealing the carbon content was reduced to 0.002 to 0.004%.

forcibly produced cm. by dipping it in a 1% H2804 solution at a room temperature for 4 hours,

on a test piece oi 10 x 15 after cnarncling it on its of fishscales (mechanical capped steel)] Two-coat enameling Degree of One-coat enameling, non-metal- Pickling at, C. Pickling at, C. pickling at, 00 C. After delie inclusion Ladle analysis, percent carburizing cleanliness, Pickling Number Pickling Number Pickling Number annealing, d 00 x 400 loss, or fishloss, of iishso, of fish- 0 Si Mn CS 0 percent percent mgJcxn 2 scales rug/cm." scales mgJcmfl scales 0. 07 (l. 01 0. 17 0. 011 0. 003 0. 30 2. 70 57 5. 28 2. 83 37 0. 08 0. 01 0.19 0.013 0. 002 0. 28 3. 43 39 7. 10 23 3. 41 29 0. 08 0. 01 0. 21 0. 010 0. 002 0. 24 4. 40 34 8. 15 16 4. 50 19 0. 07 0. 01 0. 15 0. 009 0. 003 0. 29 6. 30 6 10. 89 3 6. 53 7 0.09 0. 01 0. 10 0. 011 0. 002 0. 27 6. 70 1 12.10 0 7.15 0 0. 08 0. 01 0. 13 0.008 0. 002 0. 23 7. 95 0 1d. 00 0 8. 42 0 No'rn:

1. As a pickling solution for measuring pickling loss 7 volumes percent H2504 solution was applied.

of 10 x 15 cm. by dipping it in 2. Number of fishscale occurrence indicates number of fishscales a 1% H1304 solution at a room temperature In general, according to conventional opinions, nonmetallic inclusions should be avoided as being deterirnental to the mechanical properties of steel. However, from the above Tables 1 and 2 and the attached drawing it is evident, contrary to the conventional opinions, that the presence of non-metallic inclusions in adequate amounts, size and distribution is rather helpful for improving the cnamelability of steel.

forcibly produced on a test piece tor 4 hours, after enameling it on its one side only.

0.8 mm., which was then subjected to a decarburizing annealing in an open-coil decarburizing annealing furnace. For comparisons sake the steel of the present invention and a conventional I IS SPC-l steel material, which was annealed for decarburization, were each subjected to a standard one-coat enameling. A comparison of various properties of both kinds of steels is shown in the following tables.

TABLE 3A.--CHEMICAL COMPOSITION (WFIIGHT PERCENT) C after decarbu- C rization Si Mn P 8 Cu V A"-.- Sttciel of presentinven- 0.09 0.004 0.01 0.18 0.028 0.011 0. 04 0.04

on. B SPC-l decarburized 0.05 0.003 0.01 0. 31 0. 013 0. 016 0.05

steel.

TABLE 3B.MECHANICAL PROPERTIES Yield Grain Secondary Index point, Tensile Elon- Erich- Strain size working of cleankgJ strength gation, sen value, ratip number test liness mm. lrglmm. percent mm. (r) (ASTM) value 1 percent A. 16.2 31.7 60.9 11.7 1.79 7.3 0/8 0.28 B. 17.9 31.8 49.7 11.0 1.57 7.1 3/8 0.13

1 1n numerals indicating the secondary working test value the denominator designates total number of times of severe secondary workings having been applied, while the numerator number of secondary cracks occurred thereby.

TABLE 3C.TEST OF ENAMELABILITY Sagging value PEI Pickling (10" span, adherence loss C. x 10 index. Occurrence of velocity, Inln.), mm. percent fishscales l mg/cm.

the surface thereof.

EXAMPLE 3 A molten steel having a ladle composition of 0.10% C, 0.19% Mn, 0.01% Si, 0.026% P, 0.013% S, 0.04% Cu, 0.03% V, 0.03% Nb and 0.05% Ti was tapped from an oxygen top blowing converter. When pouring the molten steel into a mold to produce ingots therefrom, fine global granules or tablet or pill prepared by wetmixing pulverized slag, scum or broken brick pieces or ferro-chrome, etc. with cut wire of 0.8 mm. in size, in which the latter is made as a core of the granule and covered with the former, were thrown into the mold, in order to raise the degree of non-metallic inclusion cleanliness number. After slabbing, a slab of 120 mm. was rolled to hot-rolled coil of 3.2 mm. on a hot strip mill, in which the finishing temperature was 890 C. and the coiling temperature was 730 C. After pickling, the hotrolled coil was cold-rolled with a reduction rate of 75% to make a cold-rolled coil of 0.8 mm, which was further subjected to a decarburizing annealing in an open-coil annealing furnace. Then, the steel of the present invention and a conventional JIS SPC-l steel material which was also annealed for decarburization, were subjected to a standard one-coat enameling. The comparison of various properties between both kinds of steels are shown in the following tables.

molten steel; hot-rolling the ingot to make a steel sheet; cold-reducing the steel sheet to the final gauge; and annealing the thus obtained steel sheet and decarburizing so that the C is within the range of 0.02 to .015%.

2. The method of manufacturing a cold-rolled steel sheet as claimed in claim 1, wherein the non-metallic inclusions are silicates and sulfides.

3. The method of manufacturing a cold-rolled steel sheet claimed in claim 1, wherein a wet foreign substance prepared by mixing at least one kind of materials selected from the group consisting of pulverized slag, silica, brick, term-chrome, etc. with cut wire is added into a molten steel to regulate the degree of non-metallic inclusion cleanliness of the steel sheet to the range of 0.2 to 0.5%.

4. The method of manufacturing a cold-rolled steel sheet claimed in claim 1, wherein the step of making ingot comprises producing a mechanical capped steel ingot.

5. The method of manufacturing a cold-rolled steel sheet claimed in claim 1, wherein the finishing temperature of the hot rolling is in the range of 840 to 950 C. and the hot rolled sheet is coiled at a coiling temperature in the range of 680 to 750 C. and the reduction rate during cold-rolling is in the range of to 80%.

6. The method of manufacturing a cold-rolled steel sheet claimed in claim 1, wherein the molten steel contains less than 0.06% Cu and 0.015 to 0.035% P.

7. The method of manufacturing a cold-rolled steel sheet claimed in claim 1, wherein the molten steel contains at least one element selected from the group consisting of 0.02 to 0.05% V, 0.02 to 0.05% Nb, 0.01 to 0.20% Ti, 0.01 to 0.08% Al and 0.01 to 0.50% Zr.

TABLE 4A.CHEMICAL COMPOSITION (WEIGHT PERCENT) dceger 0081' u- C rization Si Mn P S Cu V Cr Nb Tl A Steel olpresent invention. 0.00 0. 008 0.02 0.10 0.020 0. 013 0.04 0. 03 0.04 0.03 0.05 B..- SPC1decarbu1-izedsteel. 0.05 0.003 0.01 0.31 0.013 0.010 0.04

TABLE 4B.MECHANICAL PROPE RTIES Yie Grain Secondary Index point Tensile Elon- Erich- Strain size working of cleankg, strength gation, sen value, ratio number te liness, mm. kg/mm. percent mm. (ASTM) value 1 percent See footnote 1 bottom Table 3b.

TABLE Mir-TEST OF ENAMELABILITY iiig Maximum (10 span, grain size 870 C. x 10 Pickling after enamel minutes) PEI adherence Occurrence of velocity, firing ASTM mm. index, percent fishscales nag/cm. Number 12. 2 100 4 300 7.2 84 $1 1s 1 gg lOO 381/i250 a. 1 11-2 See footnote 1 bottom Table 30.

What is claimed is: References Cited 1. A method off manufacturing ]a coldt-rollled steel slileetf, UNITED STATES PATENTS comprising manu acturmg a mo ten s cc compose o 0.06 to 0.15% c and 0.10 to 0.25% Mn as the basic com- 63g? alt. g 111128 ponents and the balance being Fe and unavoidable impuri- 65 3183'078 5/1965 Ohtali u 5 49 ties; where the degree of non-metallic inclusion cleanlie e 3,219,438 11/1965 Poole 75-56 ness, d 60 X 400, 1n the final product, as measured by the 3,248,270 4/1966 La1dman 148-12 ASTM point counting method, 1s below the range of 0.2 to 0.5, adding to said molten steel non-metallic inclusions CHARLES LOVELL, l' Examlnel" in an amount such that said degree of non-metallic in- US, Cl, X.R

clusion is within said range; making ingot from the said mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3,598,658 Dated August 10 1971 Inventor(s) KAMEO MATSUKURA and MASAMI OZAWA It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 9, line 68 for "below" read '--in-.

Coluinn 10, line 4 for "0.02" read 0.002

Signed and sealed this 18th day of April 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting; Officer Commissioner of Patents 

